gcc/configure: Fix check for assembler section merging support on Arm

gcc/configure: Fix check for assembler section merging support on Arm

In 32-bit Arm assembly, the @ character is the start of a comment so
the section type needs to use the % character instead.

configure.ac attempts to account for this difference by doing a second
try when checking the assembler for section merging support.
Unfortunately there is a bug: because the gcc_GAS_CHECK_FEATURE macro
has a call to AC_CACHE_CHECK, it will actually skip the second try
because the gcc_cv_as_shf_merge variable has already been set:

  checking assembler for section merging support... no
  checking assembler for section merging support... (cached) no

Fix by using a separate variable for the second try, as is done in the
check for COMDAT group support.

This problem was noticed because the recent binutils commit
d5cbf916be4a ("gas/ELF: also reject merge entity size being zero") caused
gas to be stricter about mergeable sections without an entity size:

configure:27013: checking assembler for section merging support
configure:27022: /path/to/as   --fatal-warnings -o conftest.o conftest.s >&5
conftest.s: Assembler messages:
conftest.s:1: Warning: invalid merge / string entity size
conftest.s: Error: 1 warning, treating warnings as errors
configure:27025: $? = 1
configure: failed program was
.section .rodata.str, "aMS", @progbits, 1
configure:27036: result: no

In previous versions of gas the conftest.s program above was accepted
and configure detected support for section merging.

See also:
https://linaro.atlassian.net/browse/GNU-1427
https://sourceware.org/bugzilla/show_bug.cgi?id=32491

Tested on armv8l-linux-gnueabihf.

gcc/ChangeLog:
	* configure.ac: Fix check for HAVE_GAS_SHF_MERGE on Arm targets.
	* configure: Regenerate.