[Object] getBuildAttributes: check e_machine. NFC

getBuildAttributes is only called for ARM/RISCV object files and
`SHT_ARM_ATTRIBUTES == SHT_RISCV_ATTRIBUTES`, so the following check
`Sec.sh_type == ELF::SHT_ARM_ATTRIBUTES || Sec.sh_type == ELF::SHT_RISCV_ATTRIBUTES`
is actually fine. But the convention is to guard such processor-specific
section type checks with an e_machine test.