WORKSPACE: update nelhage/rules_boost and abseil/abseil-cpp

[maxmouchet]

Hi,

This fixes two issues that prevents FlashRoute from building on ARM64.

ERROR: /root/.cache/bazel/_bazel_root/4113155584afb855b97913f42db88493/external/boost/BUILD.bazel:1583:14: Configurable attribute "defines" doesn't match this configuration (would a default condition help?).

Fixed by upgrading to the latest commit of nelhage/rules_boost, which includes https://github.com/nelhage/rules_boost/pull/168.

In file included from external/com_google_absl/absl/random/internal/randen_hwaes.cc:229:
/usr/lib/gcc/aarch64-linux-gnu/9/include/arm_neon.h: In function 'Vector128 {anonymous}::AesRound(const Vector128&, const Vector128&)':
/usr/lib/gcc/aarch64-linux-gnu/9/include/arm_neon.h:12452:1: error: inlining failed in call to always_inline 'uint8x16_t vaesmcq_u8(uint8x16_t)': target specific option mismatch
12452 | vaesmcq_u8 (uint8x16_t data)
      | ^~~~~~~~~~

Fixed by upgrading to Abseil LTS March 2021.

[lambdahuang]

LGTM, thanks for working on this!