[DebugInfo][ARM] Not readonly check for RWPI globals

When compiling for the RWPI relocation model [1], the debug information
is wrong for readonly global variables.

Writable global variables are accessed by the static base register (R9
on ARM) in the RWPI relocation model.  This is being correctly generated

Readonly global variables are not accessed by the static base register
in the RWPI relocation model. This case is incorrectly generating the
same debugging information as for writable global variables.

References:
[1] ARM Read-Write Position Independence: https://github.com/ARM-software/abi-aa/blob/main/aapcs32/aapcs32.rst#read-write-position-independence-rwpi

Differential Revision: https://reviews.llvm.org/D126361