Ensure that debug information is retained for ARMv8-M security functions.

Ensure that debug information is retained for ARMv8-M security functions.

Consider a file containing only Armv8-M secure entry functions.
This file is compiled and linked with "-march=armv8-m.main -mfloat-abi=hard
-mfpu=fpv5-sp-d16 -mcmse -static --specs=rdimon.specs
-Wl,--section-start,.gnu.sgstubs=0x190000 -ffunction-sections
-fdata-sections
-Wl,--gc-sections -g" options to generate an executable.

The executable generated does not contain any debug information of these
secure entry functions even though it contains secure entry functions in
the .text section.  This patch fixes this problem.