Look for fesetround() in libm, not in the Julia executable

Because we now do not link against libm in the executable, we must
explicitly pass `Base.libm_name` to `ccall()`.  In order to do this, we
need to re-arrange a few pieces of `base/Base.jl`, including creating a
`strcat()` from closer to first principles than we'd like.

base/Base.jl

@@ -115,6 +115,20 @@ using .Iterators: Flatten, Filter, product  # for generators
 
 include("namedtuple.jl")
 
+# For OS specific stuff
+# We need to strcat things here, before strings are really defined
+function strcat(x::String, y::String)
+    out = ccall(:jl_alloc_string, Ref{String}, (Csize_t,), Core.sizeof(x) + Core.sizeof(y))
+    GC.@preserve x y out begin
+        out_ptr = unsafe_convert(Ptr{UInt8}, out)
+        unsafe_copyto!(out_ptr, unsafe_convert(Ptr{UInt8}, x), Core.sizeof(x))
+        unsafe_copyto!(out_ptr + Core.sizeof(x), unsafe_convert(Ptr{UInt8}, y), Core.sizeof(y))
+    end
+    return out
+end
+include(strcat((length(Core.ARGS)>=2 ? Core.ARGS[2] : ""), "build_h.jl"))     # include($BUILDROOT/base/build_h.jl)
+include(strcat((length(Core.ARGS)>=2 ? Core.ARGS[2] : ""), "version_git.jl")) # include($BUILDROOT/base/version_git.jl)
+
 # numeric operations
 include("hashing.jl")
 include("rounding.jl")
@@ -163,10 +177,6 @@ include("strings/basic.jl")
 include("strings/string.jl")
 include("strings/substring.jl")
 
-# For OS specific stuff
-include(string((length(Core.ARGS)>=2 ? Core.ARGS[2] : ""), "build_h.jl"))     # include($BUILDROOT/base/build_h.jl)
-include(string((length(Core.ARGS)>=2 ? Core.ARGS[2] : ""), "version_git.jl")) # include($BUILDROOT/base/version_git.jl)
-
 # Initialize DL_LOAD_PATH as early as possible.  We are defining things here in
 # a slightly more verbose fashion than usual, because we're running so early.
 const DL_LOAD_PATH = String[]

base/rounding.jl

@@ -151,8 +151,8 @@ See [`RoundingMode`](@ref) for available modes.
 """
 :rounding
 
-setrounding_raw(::Type{<:Union{Float32,Float64}}, i::Integer) = ccall(:fesetround, Int32, (Int32,), i)
-rounding_raw(::Type{<:Union{Float32,Float64}}) = ccall(:fegetround, Int32, ())
+setrounding_raw(::Type{<:Union{Float32,Float64}}, i::Integer) = ccall((:fesetround, Base.libm_name), Int32, (Int32,), i)
+rounding_raw(::Type{<:Union{Float32,Float64}}) = ccall((:fegetround, Base.libm_name), Int32, ())
 
 rounding(::Type{T}) where {T<:Union{Float32,Float64}} = from_fenv(rounding_raw(T))