gas: sframe: fix handling of .cfi_def_cfa_register

gas: sframe: fix handling of .cfi_def_cfa_register

Fix PR gas/32879 sframe: Assembler internal error when translating
cfi_def_cfa_register

As per the documentation, .cfi_def_cfa_register modifies a rule for
computing CFA; the register is updated, but the offset remains the same.

While translating .cfi_def_cfa_register into SFrame context, we use the
information from last translated FRE to set the CFA offset. However,
there may be cases when the last translated FRE is empty.  Use last FRE
only if available.

Signed-off-by: Claudiu Zissulescu <claudiu.zissulescu-ianculescu@oracle.com>
Signed-off-by: Indu Bhagat <indu.bhagat@oracle.com>