include: move generic riscv64 ISA to rv64gc

include: move generic riscv64 ISA to rv64gc

The current CFLAGS (rv64imafdc) for the riscv64 targets do not contain
the full generic compute extension (g), as that also includes the
zicsr and zifencei extensions/instructions. Rename the default ISA to
'generic' to add distinction to the current binaries (although it's very
minimal), and use rv64gc for CFLAGS.

This is also a prep step for the upcoming gcv (vector-extension supporting)
targets like the Spacemit K1, and the thead-cores like the TH1520.

Compile-tested: all riscv64 targets
Runtime-tested:
 - SiFive Unleashed (FU540)
 - SiFive Unmatched (FU740)
 - Nezha D1 (D1)
 - VisionFive2 (JH7110)

Link: https://github.com/openwrt/openwrt/pull/18094
Tested-by: Chuanhong Guo <[email protected]> # siflower target
Signed-off-by: Zoltan HERPAI <[email protected]>