[arm] Generate a -mfpu= option for passing to the assembler

[arm] Generate a -mfpu= option for passing to the assembler

When gcc runs with the new -mfpu=auto option (either explicitly or
when that's the default behaviour) then this option is not passed
through to the assembler as we cannot rely on the assembler
understanding it (currently it doesn't understand it at all, but in
future that might change).  That means that the assembler falls back
to its builtin default, which may not correspond to what the user
expected based on the command-line options they passed.

Normally that wouldn't matter because assembler files generated by the
compiler will contain explicit directives that set the FPU type
directly and override any internal defaults; but when the compiler
driver is used to invoke the assembler directly (because the source
file ends in .s or .S) then this might cause a problem if that assumes
the FPU matches the compiler.

To address this, this patch makes the driver construct a -mfpu= option
for the assembler in the same way as the compiler generates an
internal .fpu directive.  As mentioned, this makes no difference if
the assembler file explicitly overrides the command line options, but
helps in the case where this is implicit.

	* config/arm/arm.h (arm_asm_auto_mfpu): Declare.
	(ASM_CPU_SPEC_FUNCTIONS): Add new rule asm_auto_mfpu.
	(ASM_CPU_SPEC): Use it if -mfpu is set to auto.
	* common/config/arm/arm-common.c (arm_asm_auto_mfpu): New function.
-- This line, and those below, will be ignored--

M    gcc/ChangeLog
M    gcc/common/config/arm/arm-common.c
M    gcc/config/arm/arm.h

From-SVN: r255502