testsuite: Define and use gdb_target_symbol_prefix_flags_asm.

testsuite: Define and use gdb_target_symbol_prefix_flags_asm.

Some of the source code for the test cases in the GDB testsuite
reside in .S files containing assembly code.  These files typically
define a symbol - such as main - which may, depending on the target,
require a prefix such as underscore.

For example, gdb.dwarf2/dw-compdir-oldgcc.S defines the symbol main:

main:	.globl main

Some targets, such as rx-elf, require main to have an underscore
prefix.  (If it doesn't, a linker error results due to not being able
to find _main required by crt0.o.) So, instead, the above should look
like this for rx-elf and other targets with this same requirement:

_main:	.globl	_main

This patch defines a new tcl proc in lib/gdb named
gdb_target_symbol_prefix_flags_asm.  This proc returns a string
which will - assuming everything else is wired up correctly - cause
-DSYMBOL_PREFIX=_ to be passed on the command line to the compiler.

The test cases are augmented with a macro definition for SYMBOL
as follows:

    #define CONCAT1(a, b) CONCAT2(a, b)
    #define CONCAT2(a, b) a ## b

    #ifdef SYMBOL_PREFIX
    # define SYMBOL(str)     CONCAT1(SYMBOL_PREFIX, str)
    #else
    # define SYMBOL(str)     str
    #endif

Symbols, such as main shown in the example earlier are then wrapped
with SYMBOL like this:

SYMBOL(main):	.globl SYMBOL(main)

The net effect will be to add a prefix for those targets which need
it and add no prefix for those targets which do not.

It should be noted that there was already a proc in lib/gdb.exp
called gdb_target_symbol_prefix_flags.  It still exists, but has
been significantly rewritten.  (There is only one small difference
between the two versions.)

That proc used to explicitly list targets which were known to
require an underscore prefix.  This is no longer done; the recently
added proc, gdb_target_symbol_prefix, is now invoked to dynamically
discover whether or not a prefix is required for that particular
target.

The difference between gdb_target_symbol_prefix_flags_asm
and gdb_target_symbol_prefix_flags is that the former returns
a bare prefix while the latter returns the prefix enclosed in
double quotes.  I.e. assuming that the discovered prefix is
underscore, gdb_target_symbol_prefix_flags_asm returns:

    additional_flags=-DSYMBOL_PREFIX=_

while gdb_target_symbol_prefix_flags returns:

    additional_flags=-DSYMBOL_PREFIX="_"

The double-quoted version is not suitable for using with .S files
containing assembly code; there is no way to strip the double quotes
using C preprocessor constructs.

It would be possible to use the bare (non double quoted) version in
C source code.  However, the supporting macros become more complicated
and therefore more difficult to maintain.

gdb/testsuite/ChangeLog:

	* lib/gdb (gdb_target_symbol_prefix_flags_asm): New proc.
	(gdb_target_symbol_prefix_flags): Define in terms of _asm
	version.
	* gdb.arch/i386-float.exp, gdb.arch/i386-permbkpt.exp,
	gdb.dwarf2/dw2-canonicalize-type.exp,
	gdb.dwarf2/dw2-compdir-oldgcc.exp, gdb.dwarf2/dw2-minsym-in-cu.exp,
	gdb.dwarf2/dw2-op-stack-value.exp, gdb.dwarf2/dw2-unresolved.exp,
	gdb.dwarf2/fission-reread.exp, gdb.dwarf2/pr13961.exp: Use flags
	provided by gdb_target_symbol_prefix_flags_asm.
	* gdb.dwarf2/dw2-canonicalize-type.S, gdb.dwarf2/dw2-compdir-oldgcc.S,
	testsuite/gdb.dwarf2/dw2-minsym-in-cu.S,
	testsuite/gdb.dwarf2/dw2-unresolved-main.c,
	testsuite/gdb.dwarf2/dw2-unresolved.S, gdb.dwarf2/fission-reread.S,
	gdb.dwarf2/pr13961.S: Define and use SYMBOL macro (and supporting
	macros where needed).  Use this macro for symbols which require
	the prefix provided by SYMBOL_PREFIX.