[R] Enable jemalloc in autobrew formula

[asfimport]

See

arrow/dev/tasks/homebrew-formulae/autobrew/apache-arrow.rb

Line 47 in 59a6788

-DARROW_JEMALLOC=OFF

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

PRs and other links:

• GitHub Pull Request #6073

_{Note: This issue was originally created as ARROW-6979. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
I'd like to push to make this happen for the release. Plenty of code paths will become 2x faster just by having jemalloc included.

[asfimport]

Wes McKinney / @wesm:
I'll let you when the static lib splicing work is ready

[asfimport]

Neal Richardson / @nealrichardson:
Doing in ARROW-7605