mbrola: fix cross compilation

[NickCao]

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • riscv64-linux (cross)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[davidak]

I see it fails without this change:

...
gcc -ansi -pedantic -DLITTLE_ENDIAN -O6 -IParser -IStandalone -IMisc -ILibOneChannel -ILibMultiChannel -IEngine -IDatabase -Wall -DROMDATABASE_STORE -DROMDATABASE_INIT -DSIGNAL  -o Bin/Standalone/Standalone/synth.o -c Standalone/synth.c
/nix/store/v1d8l3zqnia3hccqd0701szhlx22g54z-bash-5.1-p8/bin/bash: line 1: gcc: command not found
make: *** [Makefile:150: Bin/Standalone/Standalone/synth.o] Error 127
builder for '/nix/store/5vlkm9r3hdyzgfkivymb3vxq5dbxa0i2-mbrola-riscv64-unknown-linux-gnu-3.3.drv' failed with exit code 2
error: build of '/nix/store/5vlkm9r3hdyzgfkivymb3vxq5dbxa0i2-mbrola-riscv64-unknown-linux-gnu-3.3.drv' failed

and works with this change:

[davidak@gaming:~/code/nixpkgs]$ nix-build . -A pkgsCross.riscv64.mbrola
...
post-installation fixup
shrinking RPATHs of ELF executables and libraries in /nix/store/bv00bp3k71q8p8gp712n83d914kq31iq-mbrola-riscv64-unknown-linux-gnu-3.3
shrinking /nix/store/bv00bp3k71q8p8gp712n83d914kq31iq-mbrola-riscv64-unknown-linux-gnu-3.3/bin/mbrola
riscv64-unknown-linux-gnu-strip is /nix/store/n4pyn69crs68zmidxhacmmsqajs982jn-riscv64-unknown-linux-gnu-stage-final-gcc-debug-wrapper-10.3.0/bin/riscv64-unknown-linux-gnu-strip
stripping (with command riscv64-unknown-linux-gnu-strip and flags -S) in /nix/store/bv00bp3k71q8p8gp712n83d914kq31iq-mbrola-riscv64-unknown-linux-gnu-3.3/bin
patching script interpreter paths in /nix/store/bv00bp3k71q8p8gp712n83d914kq31iq-mbrola-riscv64-unknown-linux-gnu-3.3
checking for references to /build/ in /nix/store/bv00bp3k71q8p8gp712n83d914kq31iq-mbrola-riscv64-unknown-linux-gnu-3.3...
/nix/store/bv00bp3k71q8p8gp712n83d914kq31iq-mbrola-riscv64-unknown-linux-gnu-3.3