arm: Allow to override location of .gnu.sgstubs section

arm: Allow to override location of .gnu.sgstubs section

Depending on the DejaGNU board definition, the .gnu.sgstubs section
might be placed on different locations in order to suit the target.
With this patch, the start location of the section is overrideable
from the board definition with the fallback of the previously
hardcoded location.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/cmse/bitfield-1.c: Use overridable location.
	* gcc.target/arm/cmse/bitfield-2.c: Likewise.
	* gcc.target/arm/cmse/bitfield-3.c: Likewise.
	* gcc.target/arm/cmse/cmse-20.c: Likewise.
	* gcc.target/arm/cmse/struct-1.c: Likewise.
	* gcc.target/arm/cmse/cmse.exp (cmse_sgstubs): New.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>