ghc: allow building with current Sphinx

[chexum]

./srcpkg build ghc fails with current tools:

"inplace/bin/ghc-stage1" [...] /build/cbits/longlong.p_o
Running Sphinx v4.0.2

Configuration error:
There is a syntax error in your configuration file: (unicode error) 'unicodeescape' codec can't decode bytes in position 1-2: truncated \uXXXX escape (conf.py, line 87)

...
make: *** [Makefile:128: all] Error 2

According to https://gitlab.haskell.org/ghc/ghc/-/issues/19962

This fix is to use raw strings, as already done in master:
https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/users_guide/conf.py#L96

Sanitized from commit (only small parts apply)
https://gitlab.haskell.org/ghc/ghc/-/commit/83407ffc7acc00cc025b9f6ed063add9ab9f9bcc

General

• This is a new package and it conforms to the quality requirements

Have the results of the proposed changes been tested?

• I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
• I generally don't use the affected packages but briefly tested this PR

[skip CI] build might take over 3 hours with a single CPU - local build/pkg tested

Does it build and run successfully?

(Please choose at least one native build and, if supported, at least one cross build. More are better.)

• I built this PR locally for my native architecture, (x86_64)
• I built this PR locally for these architectures (if supported. mark crossbuilds):
  • aarch64-musl
  • armv7l
  • armv6l-musl

[chexum]

By the way, it seems the build checks are failing because there is no check target:

2021-07-10T22:39:46.1203634Z ===--- building final phase
2021-07-10T22:39:46.1204570Z make --no-print-directory -f ghc.mk phase=final check
2021-07-10T22:39:49.0161381Z make[1]: *** No rule to make target 'check'. Stop.
2021-07-10T22:39:49.0186932Z make: *** [Makefile:128: check] Error 2
2021-07-10T22:39:49.0191464Z => ERROR: ghc-8.8.4_3: do_check: '${make_cmd} ${make_check_args} ${make_check_target}' exited with 2
2021-07-10T22:39:49.0193972Z => ERROR: in do_check() at common/build-style/gnu-configure.sh:33
2021-07-10T22:39:49.0462777Z ##[error]Process completed with exit code 1.

[ericonr]

Nice find, thanks!