community/gcc-cross-embedded: enable support for additional RISC-V ext.

I use Alpine in conjunction with a RISC-V simulator. This simulator also supports the more niche extensions such as the E-Extension. Unfortunately, the RISC-V cross toolchain we are shipping presently does not support many of the less mainstream -march/-mabi combinations. This MR enables more multilibs for our RISC-V cross toolchain based on the combinations that Debian supports. There should really be no downside to supporting more of these apart from a minimally increased compilation time and package size.

Without this MR:

$ riscv-none-elf-gcc --print-multi-lib
rv32i/ilp32;@march=rv32i@mabi=ilp32
rv32im/ilp32;@march=rv32im@mabi=ilp32
rv32iac/ilp32;@march=rv32iac@mabi=ilp32
rv32imac/ilp32;@march=rv32imac@mabi=ilp32
rv32imafc/ilp32f;@march=rv32imafc@mabi=ilp32f
rv64imac/lp64;@march=rv64imac@mabi=lp64
rv64imafdc/lp64d;@march=rv64imafdc@mabi=lp64d

With this MR:

$ riscv-none-elf-gcc --print-multi-lib
rv32e/ilp32e;@march=rv32e@mabi=ilp32e
rv32ea/ilp32e;@march=rv32ea@mabi=ilp32e
rv32em/ilp32e;@march=rv32em@mabi=ilp32e
rv32eac/ilp32e;@march=rv32eac@mabi=ilp32e
rv32emac/ilp32e;@march=rv32emac@mabi=ilp32e
rv32i/ilp32;@march=rv32i@mabi=ilp32
rv32ia/ilp32;@march=rv32ia@mabi=ilp32
rv32im/ilp32;@march=rv32im@mabi=ilp32
rv32if/ilp32f;@march=rv32if@mabi=ilp32f
rv32ifd/ilp32d;@march=rv32ifd@mabi=ilp32d
rv32iaf/ilp32f;@march=rv32iaf@mabi=ilp32f
rv32iafd/ilp32d;@march=rv32iafd@mabi=ilp32d
rv32imf/ilp32f;@march=rv32imf@mabi=ilp32f
rv32imfd/ilp32d;@march=rv32imfd@mabi=ilp32d
rv32iac/ilp32;@march=rv32iac@mabi=ilp32
rv32imac/ilp32;@march=rv32imac@mabi=ilp32
rv32imafc/ilp32f;@march=rv32imafc@mabi=ilp32f
rv64i/lp64;@march=rv64i@mabi=lp64
rv64ia/lp64;@march=rv64ia@mabi=lp64
rv64im/lp64;@march=rv64im@mabi=lp64
rv64if/lp64f;@march=rv64if@mabi=lp64f
rv64ifd/lp64d;@march=rv64ifd@mabi=lp64d
rv64iaf/lp64f;@march=rv64iaf@mabi=lp64f
rv64iafd/lp64d;@march=rv64iafd@mabi=lp64d
rv64imf/lp64f;@march=rv64imf@mabi=lp64f
rv64iac/lp64;@march=rv64iac@mabi=lp64
rv64imac/lp64;@march=rv64imac@mabi=lp64
rv64imafc/lp64f;@march=rv64imafc@mabi=lp64f
rv64imafdc/lp64d;@march=rv64imafdc@mabi=lp64d

---

CC: @maribu