arm: Support -mfdpic for more targets

arm: Support -mfdpic for more targets

Targets that are not arm*-*-uclinuxfdpiceabi can use -S -mfdpic, but -c
-mfdpic does not pass --fdpic to gas.  This is an unnecessary
restriction.  Just define the ASM_SPEC in bpabi.h.

Additionally, use armelf[b]_linux_fdpiceabi emulations for -mfdpic in
linux-eabi.h.  This will allow a future musl fdpic port to use the
desired BFD emulation.

gcc/ChangeLog:

	* config/arm/bpabi.h (TARGET_FDPIC_ASM_SPEC): Transform -mfdpic.
	* config/arm/linux-eabi.h (TARGET_FDPIC_LINKER_EMULATION): Define.
	(SUBTARGET_EXTRA_LINK_SPEC): Use TARGET_FDPIC_LINKER_EMULATION
	if -mfdpic.