S/390: Add direct support for Linux kernel __fentry__ patching.

S/390: Add direct support for Linux kernel __fentry__ patching.

On i386, the difference between mcount and fentry is that fentry
comes before the prolog. On s390 mcount already comes before the
prolog, but takes 4 instructions. This patch introduces the more
efficient implementation (just 1 instruction) and puts it under
-mfentry flag.

The produced code is compatible only with newer glibc versions,
which provide the __fentry__ symbol and do not clobber %r0 when
resolving lazily bound functions. Because 31-bit PLT stubs assume
%r12 contains GOT address, which is not the case when the code runs
before the prolog, -mfentry is allowed only for 64-bit code.

Also, code compiled with -mfentry cannot be used for the nested C
functions, since they both use %r0. In this case instrumentation is
not insterted, and a new warning is issued for each affected nested
function.

2018-07-16  Ilya Leoshkevich  <iii@linux.ibm.com>

	* common.opt: Add the new warning.
	* config/s390/s390.c (s390_function_profiler): Emit "brasl
	%r0,__fentry__" when -mfentry is specified.
	(s390_option_override_internal): Disallow -mfentry for 31-bit
	CPUs.
	* config/s390/s390.opt: Add the new option.

2018-07-16  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/mfentry-m64.c: New testcase.

From-SVN: r262732