riscv: Fix detecting FPU support in standard extension

riscv: Fix detecting FPU support in standard extension

We should check the string until it hits underscore, in case it
searches multi-letter extensions. For example, "rv64imac_xandes"
will be treated as D extension support since there is a "d" in
"andes", resulting illegal instruction caused by initializing FCSR.

Signed-off-by: Yu Chien Peter Lin <[email protected]>
Reviewed-by: Rick Chen <[email protected]>
Reviewed-by: Padmarao Begari <[email protected]>
Reviewed-by: Samuel Holland <[email protected]>