gdb/risc-v: fix ISA string detection for disassembly

gdb/risc-v: fix ISA string detection for disassembly

Commit 3f61a38 introduced a regression where the ISA string was no
longer detected based on the ELF header. The mechanism was changed from
directly referencing `abfd` to using `disassembler_info->section`, which
was not properly initialized for RISC-V.

The previous implementation ignored the object in scope, leading to
issues such as failing to decode RVV instructions when a library was
compiled as `rv64gcv` and the main application as `rv64gc`.

This patch resolves both problems by initializing
`disassembler_info->section` with the object currently in scope,
ensuring correct ISA string detection during disassembly.

Approved-By: Andrew Burgess <aburgess@redhat.com>