revert sanitizer back to the version we were using with GCC 9, since
that one was already newer than the GCC 10 version.

initial import of GCC 9.3.0. changes include:

- live patching support
- shell completion help
- generally better diagnostic output (less verbose/more useful)
- diagnostics and optimisation choices

initial import of GCC 9.3.0. changes include:

- live patching support
- shell completion help
- generally better diagnostic output (less verbose/more useful)
- diagnostics and optimisation choices can be emitted in json
- asan memory usage reduction
- many general, and specific to switch, inter-procedure,
profile and link-time optimisations. from the release notes:
"Overall compile time of Firefox 66 and LibreOffice 6.2.3 on
an 8-core machine was reduced by about 5% compared to GCC 8.3"
- OpenMP 5.0 support
- better spell-guesser
- partial experimental support for c2x and c++2a
- c++17 is no longer experimental
- arm AAPCS GCC 6-8 structure passing bug fixed, may cause
incompatibility (restored compat with GCC 5 and earlier.)
- openrisc support

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import GCC 8.3. it includes these new features:
- many optimisations improved: inter-procedural, profile-directed,
LTO, loops including user-controllable unroll support, and more.
- columns number

import GCC 8.3. it includes these new features:
- many optimisations improved: inter-procedural, profile-directed,
LTO, loops including user-controllable unroll support, and more.
- columns numbers added to line numbers in dwarf
- gcov extended significantly
- many sanitizer updates
- many new warning messages
- many better hints and more useful error messages
- minor ABI changes on x86-64 libstdc++, and some c++17 modes
- draft c++2a features
- better c++17 experimental support
- Armv8.4-A supported, better 8.2-A and 8.3-A support, including
32 bit arm port. cortex a-55, a-75 and a-55.a-75 combo support.
- in the GCC bugzilla, 8.1 shows 1149 bugs fixed, 8.2 shows 100, and
8.3 shows 158.

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