custom asm mnemonics unrecognized in current riscv-gnu-toolchain

For example, the following assembler statement should translate to the opcode 0x0000000b

custom0 0,0,0,0

With an older version of riscv-gnu-toolchain (built on Jun 4th) no -march= option was needed to assemble this. However, with todays git head of riscv-tools I get:

$ /opt/riscv_new/bin/riscv64-unknown-elf-as -m32 test.s
test.s: Assembler messages:
test.s:1: Error: unrecognized opcode `custom0 0,0,0,0'

From reading the source code I'd guess passing the arch Xcustom to the tools should fix that, but:

$ /opt/riscv_new/bin/riscv64-unknown-elf-as -m32 -march=RV32IMXcustom test.s 
test.s: Assembler messages:
test.s:1: Error: unrecognized opcode `custom0 0,0,0,0'

(At the moment the PicoRV32 test firmware does not build with the latest riscv-gnu-toolchain because of this.)

Edit: The last version of riscv-tools that work for me is commit 84a47e0b4e from Aug 20. With that version the custom0 opcode is recognized with and without the -march= option.

Sorry this took so long to find -- I don't read stackoverflow. For future reference, if you submit a github issue/PR or email me directly, I'll deal with these sorts of things.

A fix should be here, do you mind verifying it? https://github.com/riscv/riscv-gnu-toolchain/pull/97

Sorry for introducing the regression!