Disable Clang's integrated assembler for two testcases

Disable Clang's integrated assembler for two testcases

gdb.dwarf2/dw2-dir-file-name.exp fails to build using Clang because
the generated assembly language contains .ascii directives with more
than one string literal.  gdb.dwarf2/dw2-restore.exp fails to build
using Clang because it contains .func and .endfunc directives.
This commit causes Clang to invoke the system assembler to assemble
the relevant files.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-dir-file-name.exp: Use system assembler
	when compiling with clang.
	* gdb.dwarf2/dw2-restore.exp: Likewise