Avoid stat-ing stdlib path if it's unreadable

[giordano]

This seems to fix #55966 for me on v1.10:

julia> run(`sudo mkdir -p $(abspath(Sys.BUILD_STDLIB_PATH))`);

julia> run(`sudo chmod go-x $(abspath(Sys.BUILD_STDLIB_PATH))`);

julia> using SuiteSparse
[ Info: Precompiling SuiteSparse [4607b0f0-06f3-5cda-b6b1-a6196a1729e9]

julia>

It's hard to make a proper test for this because it requires fiddling with the build directory, which for from-source builds is likely needed and you don't want to mess with it, and in general may require admin access.

I'll make a separate PR for master, as sketched in #55966 (comment), I started from v1.10 because that's the only version where I can reliably reproduce the issue, and so test it locally easily. Also, the code around these lines is changed in v1.11, so this would need a manual backport in any case.

[giordano]

Note that the core testset is bound to fail because it's currently broken on the target branch.

[fatteneder]

loading.jl contains many more fs calls that run through stat and could potentially fail the same way.
I guess we need to make more use of isaccessibledir and friends to filter such cases prematurely (but that's also not the case here).